Dabbler - 13-7-2018 at 20:32
I soaked an unknown green mineral in HCl thinking it was a copper carbonate, but found instead that the green was from Iron II in a matrix of
something insoluble in acid or base. There were also unidentified black crystals in some samples. The dissolved matter precipitated Iron III
oxide-hydroxide when basified with NaOH and oxidized with peroxide.
The mystery is that the clear basic supernatant left over (after iron oxide-hydroxide) will precipitate copious amounts of a pure white compound with
addition of 30% Hydrogen peroxide. That precipitate re-dissolves slowly if the solution is strongly basic, but when the solution is less basic it can
be filtered out to a white paste which dries to a white cement. The material does not melt in a butane flame, but shrinks and powders.
It is not sodium silicate since the filtrate will still precipitate silica and ultimately gel if fully neutralized with HCl and without adding any
peroxide. I also tried adding peroxide to a pure sodium silicate solution as a control and got no precipitate at all.
Can anyone guess what this compound is? Are there further tests I can try?
j_sum1 - 13-7-2018 at 22:21
I started off with an unknown substance and did a large number of things to it and still don't know what it is...
You need to simplify your processes and be systematic.
Go back to your original substance. Find out what acids dissolve all or part of it.
Once you have a solution, test systematically for cations. How do you know that Fe is present? Testing with ferrocyanide and ferricyanide will tell
you. Flame tests on your original substance and also on your solutions would also be good.
You might also inform us as to the origin of the substance. If it is from a rock then I would expect some kind of aluminosilicates. And good luck
getting further than that with simple amateur lab tests. Colours in this case can arise from minute quantities of other elements and therefore colour
is not a reliable indicator of anything.
Dabbler - 17-7-2018 at 18:48
Hi.
The original substance tests positive for Iron II with Potassium Ferricyanide, and the acid leach (basified by NaOH and oxidized with H2O2) tests
positive with Ferrocyanide, confirming that the green color is from Iron.
The clear supernatant (after settling out the Iron oxide-hydroxide from the HCl leaching) formed a white solid with the addition of H2O2. After
drying the solid at 50C most of it disappeared, leaving a soluble basic compound and an insoluble white foam.
My guess about this mystery compound is that it is a mix of silicate & (maybe aluminates) with sodium peroxide octahydrate. That is just an
artifact of excess NaOH in the supernatant. Likely the sodium peroxide octahydrate decomposed upon heating and drying into NaOH and H2O2 which
evaporated. Test with NaOH and Peroxide does produce a solid that decomposes over a day or so.